Method of making magnetic cores for transformers or the like



Jan. 25, 1955 MSKY 2,700,207

J. J. Zl METHOD OF MAKING MAGNETIC CORES FOR TRANSFORMERS OR THE LIKE Filed Aug. 2, 1952 2 Sheets-Sheet l Fig. 5.

I INVENTOR.

John .J. Z imsky United States Patent METHGD (BF MAKENG MAGNETIC CORES FOR TRANSEF OR THE LIKE John .1. Zirnsky, Canonshurg, Pa., assignor to McGraw Eicctric Company, a corporation of Delaware Application August 2, 1952, Serial No. 302,299 7 Claims. (Cl. 29-15558) This invention relates to a new method for making preformed magnetic cores for transformers or the like in which the individual turns of each core have their respective ends in lap joint relation. More particularly, this invention pertains to a combination of steps for making such cores without rebuilding.

In the construction of cores for magnetic induction equipment, it is common practice to use thin ferrous cold-rolled strip. in the rolling of such strip to thinner gauges such as .014 inches, for example, the grain of the metal is oriented and offers a lower reluctance magnetic flux flow path through the metal in the direction of the rolling. This advantage increased the use, particularly for smaller transformers such as those in the so-called distribution class, of wound-type cores in which the strip is bent lengthwise so that the plane of the metal surface is generally parallel to the axis of the core about which the metal is wound or formed in equivalent fashion. Generally, such wound-type cores fall into a number of categories. Where unbroken spiral wound cores are employed, the conductive winding or windings have to be woundand linked with the core in situ giving rise to numerous disadvantages. In another kind of wound core, butt joints are employed with higher magnetic joint reluctance and higher no-load losses. In other wound cores, lap joints are employed decreasing the joint reluctance but in suchcases it has been considered desirable to rebuild the core in the course of construction or to begin the construction with core strips of varying length or to employ a multitude of joints in the respective turns. In those cases where the cores have been cut in the course of constructing the joints, annealing following such cutting has been considered desirable to ameliorate the core deterioration effect of the cutting. Further, in the higher rated distribution transformer classes such as those from 167 kva. and above and in the so-called power transformer classes from 00 kva. and above, many manufacturers have considered it impractical to employ woundtype cores in those services.

By means of my new method, magnetic cores of the wound kind for transformers or the like may be constructed for use, for example, in transformers of ratings above 167 kva. In addition, by means of the new method, no rebuilding of the core is required and the advantages of lap joints and of single joints per turn of the core may be retained. Since in the linking of preformed magnetic cores made by my new method with preformed windings, no bending of the core turns is required beyond the elastic limit of the material, no impairment of the magnetic properties of the core results. Other objects and advantages of this invention will be apparent from the following description and from the drawings, which are illustrative only, in which Figure 1 is an end view of a core in the course of being wound in one practice of the new method of this invention;

Figure 2 is a side view of the core shown in Figure 1 following completion of the winding thereof and its banding, the core being in a position on the floor of an annealing furnace preparatory to annealing;

Figure 3 is a view of the core shown in Figure 2 in the course of having spacers removed therefrom;

Figure 4 is a partial view in side elevation of the an nealed core shown in Figure 3 after the spacers and band have been removed therefrom and the spaced core end clamped for a cutting operation which is shown taking place;

a filler and mold block Figure 5 is a plan view of the elements shown in Figure 4 after the cutting wheel has been withdrawn from the cutting stroke;

Figure 6 illustrates a modified shell-type transformer in the act of having the winding or windings linked with turns of a core made in accordance with a practice of this invention; and

Figure 7 is a view of a modified core-type transformer made with the aid of a core constructed in accordance with a practice of this invention.

Referring to the drawings, a polygonal mandrel 10, which may be in the form of a four-sided elongated keystone, may be provided. Mandrel 10 is provided with a pair of longer opposite sides 11 and a pair of parallel narrower sides 12 and 13, side 12 being the wider in the illustrated practice. Mandrel 10 may be removably bolted to an arbor 14 capable of being turned by any suitable mechanism to Wind a continuous ferrous strip 15 of magnetic material around mandrel 10 in the course of the rotation of mandrel 10, for example, in the direction of the solid arrows shown in Figure 1. Although but one strip 15 is shown being wound in Figure 1, a plurality of such strips in radially superposed relation may be so wound comprising together What may be considered a continuous single length of ferrous strip. Such ferrous strip is preferably made of a suitable cold-rolled silicon electric steel having the desired magnetic properties. Such steel is usually obtained with an insulating surface thereon in the form of an oxide or other insulating coating.

in commencing the winding of a strip 15 around mandrel 10, the leading edge of strip 15 may be tack welded or otherwise clamped to mandrel 10 preferably in such a manner as to maintain the symmetry of the core steel being so wound. During the winding, a uniform tension may be maintained on strip 15 and every few turns, possibly three, a spacer plate 16 may be placed on the side of the core being constructed radially outwardly from side 12 of mandrel 10. In that way, a coil 17 being constructed from strip 15 will have each of the laminated turns in close contact with one another around the rounded corners of mandrel 10 and along the sides of 11 and 13 thereof. The sides of the turn laminations of core 17 in end 25 radially outwardly from side 12 will also be held tightly together against the adjoining lamination or against a surface of a spacer 16. The sides of the respective turns adjoining the surfaces of the spacers 16 generally parallel to side 12 may in the winding of strip 15 create relatively small gaps 18 adjacent the edges of the spacer plates 16 parallel to the axis of arbor 14 and mandrel 10, which is also the axis of core 17. The placement of the spacers 16 may readily be performed manually, each newly placed spacer being caught inwardly of the portion of strip 15 next to the spacer 16 on the outer side thereof. The spacer plates 16 may be made of preannealed mild steel plate or castings and should have no component therein or thereon which in the annealing of core 17 or at any other stage might tend to adversely affect the composition or condition of the magnetic core made from a strip 15.

When a suflicient number of turns of strip 15 has been wound around mandrel 10 to provide the desired core thickness, strip 15 may be severed at that point to provide a trailing end which can be tack welded to the outermost turn of coil 17 or completed coil 17 can be banded directly without such tack welding. A band 19 of steel of suitable composition may be atfixed around the outermost turn of coil 17 by a conventional banding machine or banding tool using a crimping seal 20, for example, to hold band 19 tight. Band 19 in turn holds coil 17 and spacer plates 16 in their respective positional relation as wound on mandrel 10, mandrel 10 remaining as for the window 21 of coil 17, as shown in Figure 2, after mandrel 10 with coil 17 thereon has been disconnected from arbor 14. The dimensions of mandrel 10 and the number of turns in a coil 17 will be predetermined in accordance with lar electrical and physical qualities which the completed core is to have. In the illustrated practice, the lines indicating turn laminations are only generally illustrative in view of the different thicknesses of steel strip and radial the particuthicknessesv of; magnetic core. that may be made under this invention. If desired, the spacer plates 16 may extend sufficiently far axially to one or both sides of coil 17 to lend themselves to being generally radially aligned by means of a guide bar fitting cooperating notches in the spacer plates, the guide bar being removably bolted to mandrel'10. Although the coil 17 illustrated in the drawings is of a uniform width, cruciform cores may readily be constructed in accordance with the method of th s invention by the winding of appropriate lengths of strip of appropriate width in series relation. The ends of the respective lengths of. the respective widths may be joined by butt welding or otherwise generally m thecourse of the winding operation so that a completed co1l of wound strip of difierent widths will have the respective lengths of those respective widths of the desired radial thickness or build and in the proper radial order relative to the axis of the core.

Wound coil 17, with mandrel 19, the spacer plates 16 and retaining band 19, may be placed levelly on edge on blocks 22 on the floor or hearth 23 of an annealing furnace or oven. Such an annealing furnace may be the form of a conventional box annealing furnace in which a cover or a plurality of covers are set down so as to cover the work to be annealed, in this case, coil 17. By placing coil 17 on edge and level on blocks 22, sagging or other deformation is prevented, mandrel 111 and spacer plates 16 being also levelly supported and in their wound relation to coil 17 in the course of the annealing operation. In the annealing which is generally conducted in an inert or other suitable gaseous atmosphere, coil 17 is heated to a sufliciently high temperature to preset coil 17 so that the annealed turns of coil 17 become preformed and will hold their generally quadrangular shape. Further, by such annealing the strains introduced mto the metal of strip 15 by the winding operat1on such as that illustrated in Figure l are removed, improving the magnetic qualities of the strip. It will be noted that the annealing step described presets a formed and shaped coil thereby achieving in the finished core a preformed quality while avoiding any rebuilding of the turns of a coil like coil 17. A great advantage therefore is that by this mvention there is no handling before annealing of separate laminations or turns of a core undergoing construction.

Following an annealing step as described, the coil 17, now preformed, may have the band 19 removed therefrom. As in' the case of the metal of mandrel 10 and of spacer plates 16, the metal of band 19 and lock 29 is selected so as not to adversely affect the magnetic n? quality of the core steel in a coil 17. Also following such annealing, mandrel 10 is removed from window 21 of coil 17 which may be placed on a workbench 24 with its end 25 extending over the end of workbench 24.

End 25 is that end which has spacer plates 16 therein so '1 that a workman may use, for example, a hammer 26 and wooden striking block 27 to drive out the spacer plates 16. In the illustrated embodiment, the build at end 25 of core 17 will remain greater, even after spacer plates 16' are driven out, than the radial thickness of the op- 53 posite end 28 of coil 17 which is now in the form of an incomplete preformed annealed core comprising a continuous spiral of one or more strips 15. Ends 25 and 28' in the finished core will comprise the yoke legs, the additional width from end to end of yoke leg 25 oocasioned by the use of spacers 16 and/or the greater width along side 12 of mandrel 10 providing sufficient length to create a lap joint in yoke leg 25 in each of the turns of the completed core 17 when the adjoining winding legs-29 are brought into parallel relation in the course of completing the construction of a transformer or the like with a core made in accordance with a practice of this invention.

Following the removal of spacer plates 16 from end 25,,a cutting machine of conventional nature embodying an adjustable pair of spaced clamps 30 and a thin abrasive cutting wheel 31 may be used in carrying forward the illustrated practice of this invention. Thus, if workbench 24 or the cutting machine is mounted on-rollers and the end 25 of core 17 projects over the end of workbench 24, the clamp arms in each pair when spread apart can be raised to radially span both sides of end 25 with. its fullbuild-up. as originally Wound. Eachpair of the clamps 30' may be spaced about an equal distance on each side of a plane normal to the laminations in end 28 and passing through the axis of core 17, that is, a plane equidistant from the respective winding legs 29. With the clamps in such a position, they can be tightened axially of and by threaded clamp standards or adjustment rods 32, bringing them together and substantially and radially closing up all of the gaps between the sides of the turns in end 25 which remained after the removal of the spacers 16. Such collapsing radially of the turn portions in end 25 reduces the thickness at that point substantially to the thickness as wound of legs 28 and 29. In addition, however, such clamping moves the respective portions of the turn laminations in legs 28 and 29 sufficiently relative to one another to cause a flexing or slight relative movement. Thereby, if there had been any tendency to develop an adhesion in the course of annealing, that tendency is negatived by the clamping action described as it also would be by the action of linking the core with one or more windings.

The cutting machine need not be further illustrated or described in view of the many alternatives available for performing the clamping and cutting operation on a coil like coil 17. It will sufiice to say, for example, that the threaded rods 32 may be free to turn with respect to the outermost of the pairs of clamp arms 30 while the innermost clamp arms, relative to the cutting machine, may have the hubs thereof internally threaded so that rotating rods 32, by means not shown, will move the innermost clamping arms axially of the rods 32 toward or away, as the case may be, from the outermost of the clamping arms. Similarly cutting wheel 31 may be mounted on a shaft 33 which is rotated at high speed by some suitable connection extending through the mounting brackets 34 supporting shaft 33. Themounting brackets 34 in turn can be caused by the machine to oscillate through an are which may extend from dotted position A to dotted position B shown in Figure 4. After the clamping by the clamps 30 the movement of cutting wheel 31 from position A to position B will cut a kerf 35 generally radially through an intermediate portion of end 25 and in so doing Will sever the turns of annealed coil 17. Such a severance will convert preformed annealed coil 17 from a continuous spiral into a nest of single whole turn laminations in a closed core each of Which turns has but a single joint in an intermediate portion of a yoke leg at end 25. When the ends of each of the resulting whole turn laminations are overlapped to form a lapped joint in substantially each of the whole turn laminations there is produced a magnetic joint of relatively low joint reluctance and high magnetic quality. In the case of transformers or the like of relatively higher ratings, joint reluctance is proportionately a smaller part of the core reluctance than in the case of magnetic cores employed in transformers or the like of lower ratings. Consequently, the disturbance of the magnetic quality of the core metal occasioned by cutting a coil 17 after preshaping it by annealing in wound shaped form is insufficient to adversely affect the performance of the core in service to an extent which makes the core impractical to use. Further, abrasive cutting wheels such as cutting wheel 31 create substantially no burr problem at the ends of the turn laminations bordering the kerf 35 so there is no shortcircuiting occasioned between turn laminations, especially since at least in the linking of the turn laminations with a winding or windings there is a sufiicient movement of the metal to shake off any powdery particles at the ends of the respective turns which may have been occasioned by the cutting step.

Following the cutting operation, clamps 36 are released and the cutting machine and the now completed preformed annealed core formed from coil 17 are moved away from each other and core 17 is ready to be linked with a winding or windings of a transformer or the like. As shown in Figure 6, preformed transformer windings 36 having a window 37 between the inner cylindrical.

sides thereof have already been linked with a preformed annealed core 33 made in accordance with a practice of this invention as illustrated in Figures 1 to 5 inclusive. Thus in core 38 the yoke ends 39 in the leg corresponding to yoke end 25, have had the ends of substantially all of the turns individually overlapped at the same end of winding 36, the respective lapped joints of the respective turns being generally in alignment in the plane of the cut made by the cutting machine in the course of constructing core 3%; Another core 41formed similarly to core 38 and to the core constructed from coil 17 is shown in the course of being linked with winding 36.

That linking is in accord with the method covered in United States patent application Serial No. 665,045, filed April 26, 1946, for Method of Making Magnetic Cores. Thus, in that linking, single turns or groups of turns from one core may be applied with the ends of the turns being linked opened, leading and abreast, as shown when moving in the direction of the arrow shown in Figure 6, there being no stressing of the turns to a point which will impair their magnetic qualities. Thus, by way of example, two whole turn core laminations 42 and 43 are being simultaneously linked in the operation shown in Figure 6. When the yoke leg side 44 and 45 of the turns 42 and 43 move into contact with the previously linked portions of yoke leg 46a of core 41, the ends 46 and 47 of the turns 42 and 43 may be respectively overlapped to create a lap joint in each of the turns thus making each of the turns a relatively independent magnetic circuit without any dependence upon butt joints. Such linking of a turn or turns from the core structure of core 41 is continued until the entire core is in place relative to winding 36 as shown in the case of core 38 which has already had completed the linking and overlapping of its turns with winding 36. The lap joints 49 of core 41 will preferably be in alignment at the same end of winding 36 as the aligned lap joints 40 of core 38. In the linking of cores with a winding or windings as shown, the linking begins with the innermost turns defining the Window of the core, like window 21 in the case of core 17, and continuing with the radially and successively outer turns in the particular core being linked. It will be noted that the turns after linking occupy the identical positions relative to one another that they occupied in the course of being wound on a mandrel and in the course of being annealed to preshape and preform them. Hence, the corners and sides of each core will fit and their high magnetic properties will remain unimpaired.

It will be evident that a core made in accordance with the practice of this invention may be so utilized that each of the legs adjoining a yoke leg comprises a winding leg capable of being linked with a preformed winding to make a modified core type transformer or the like. In such an event, the leading ends of the respective turns will both be passed through the respective windows of the respective windings, the axes of which windings will generally be parallel, to complete a structure such as that illustrated in Figure 7. Further in a construction illustrative of a practice of this invention, curved spacers may be employed in place of rectilinear spacers such as spacer plates 16, the curvature of such spacers being in the direction of the curvature of the yoke leg of the core which is to have the turn joints therein. In some cases, cores may be constructed by cutting through both yoke ends so that three half cores, for example, may be linked with their respective one-half turn ends overlapping to form a three-legged core useful in making a three phase transformer. Preferably the mandrel used in winding will be of keystone or trapezoidal form as described but in some cases, it may be rectangular when viewed along its axis, spacers in such a case being used not only to take care of the build-up occasioned by the overlapped joints but also to take care of providing sufiicient metal for such an overlap without relative movement of the winding leg sides of the core adjoining the yoke leg in which the joint is to be placed. Preferably also, the spacers and mandrel are removed before cutting but in some cases, the spacers may be left in place and the mandrel may be provided with a slot in alignment with the cutting wheel so that the cutting operation may take place without contacting or cutting the mandrel itself.

Various materials and devices other than those mentioned may be employed as will be understood by those skilled in the art. Variations in the steps for the practice of this invention may be made without departing from the spirit of this invention or the scope of the appended claims.

I claim:

1. in the method of constructing a magnetic core, the steps including winding magnetic strip material on a generally trapezoidal mandrel to form a plurality of concentric turns, concurrently with said winding step inserting spacers between selected turns on the longer parallel side of the mandrel to provide a space factor between said selected turns, holding the turns in substantially their wound shape and annealing said turns while so held to improve the magnetic qualities of the magnetic strip material and to pre-set the turns into a unitary annealed article of generally trapezoidal shape, cutting through all the turns intermediate the ends of said longer side to produce a plurality of single, discontinuous, concentrically-nested turns, inserting said turns through at least one preformed electrical winding, and overlapping the ends of said turns to provide a magnetic core with lap joints in one of the yokes.

2. The method of making magnetic induction apparatus which includes the steps of winding magnetic metal strip on a generally trapezoidal mandrel to form a plurality of turns, concurrently with said winding step and during the continued course of said winding step inserting spacers between successive selected turns on the longer parallel side of the trapezoidal mandrel to provide a space factor between the selected turns, holding the turns in substantially wound shape while annealing the turns to pre-set all of the turns into a formed and shaped trapezoidal configuration and to improve the magnetic qualities of the magnetic metal strip by relieving internal strain in the magnetic metal strip, thereby to provide a unitary annealed article having a generally trapezoidal shape, cutting through all of the turns along the longer parallel side of the annealed article to produce a trapezoidal shaped core having a plurality of single concentric discontinuous turns, inserting the turns through the window of an electrical winding, and thereafter linking the turns with the adjoining free ends of the turns substantially and appreciably overlapped to form a closed magnetic flux path and providing a rectangular magnetic core for a coil and core assembly with lap joints in one of the yoke legs.

3. in the method of constructing a magnetic core, the steps including winding magnetic strip material on a generally trapezoidal mandrel to form a plurality of concentric turns, concurrently with said winding step inserting spacers between selected turns on the longer parallel side of the mandrel to provide a space factor between said selected turns, holding the turns in substantially their wound shape and annealing said turns while so held to improve the magnetic qualities of the magnetic strip material and to pre-set the turns into a unitary annealed article of generally trapezoidal shape, clamping said turns together on the longer of the parallel sides of said generally trapezoidal shaped article, cutting through all the turns intermediate the ends of said longer side to produce a plurality of single, discontinuous concentrically nested turns, inserting said turns through at least one preformed electrical winding, and overlapping the ends of said turns to provide a magnetic core with lap joints in one of the yokes.

4. The method of making magnetic induction apparatus which includes steps of winding magnetic metal strip on a generally trapezoidal mandrel to form a plurality of turns, concurrently with said winding step and during the continued course of said winding step inserting spacers between successive selected turns on the longer parallel side of the trapezoidal mandrel to provide a space factor between the selected turns, holding the turns in substantially wound shape while annealing the turns to pre-set all of the turns into a formed and shaped trapezoidal configuration and to improve the magnetic qualities of the magnetic metal strip by relieving internal strain in the magnetic metal strip, thereby to provide a unitary annealed article having a generally trapezoidal shape, cutting through all of the turns along the longer parallel side of the annealed article to produce a trapezoidal shaped core having a plurality of single concentric discontinuous turns, spreading the trapezoidal shaped turns without producing substantial internal strain in the magnetic metal strip to separate and space the adjoining free ends of each discontinuous turn, inserting the spread turns through the window of an electrical winding, and thereafter linking the turns with the adjoining free ends of the turns substantially and appreciably overlapped to form a closed magnetic flux path and providing a rectangular magnetic core for a coil and core assembly with lap joints in one of the yoke legs.

5. The method of making magnetic induction apparatus which includes the steps of Winding magnetic metal strip on a generally trapezoidal mandrel to form a plurality of turns, concurrently with said winding step and during the continued course of said winding strip inserting spacers between successive selected turns on the longer parallel side of the trapezoidal mandrel to provide a space factor-between the selected turns, holding the turns in substantially wound shape while annealing the turns to pre-set all of the turns into a formed and shaped trapezoidal configuration and to improve the magnetic qualities of the magnetic metal strip by relieving internal strain in the magnetic metal strip, thereby to provide a unitary annealed article having a generally trapezoidal shape, cutting through all of the turns along the longer parallel side of the annealed article to produce a trapezoidal shaped core havin a plurality of single concentric discontinuous turns, spreading the trapezoidal shaped turns without producing substantial internal strain in the magnetic metal strip to separate and space the adjoining free ends of each discontinuous turn, inserting the spread turns through the window of an electrical winding, linking the turns with the adjoining free ends of the turns substantially and appreciably overlapped, whereby any adhesions formed upon annealing between successive turns along said longer parallel side will be displaced, and holding said turns in overlapped position to form a closed magnetic flux path, thereby to provide a rectangular. magnetic core fora coil and core assembly with lap joints in one of the yoke legs.

6. The method of claim 1 wherein each separate turn is individually overlapped during the step of overlapping the turns.

7. The method of claim 1 wherein selected pluralities of successive turns are collectively overlapped during the step of overlapping the turns.

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